1. Field of the Invention
The present invention relates to a positioning device for a magnetic head, and more particularly, to a magnetic head positioning device of a fixed head type magnetic recording/reproduction device.
2. Description of the Background Art
A magnetic head is used for recording and reproducing information to and from a magnetic recording medium. The magnetic head carries out recording and reproduction of information by performing an electric-magnetic or magnetic-electric converting operation according to the recorded information or reproduced information.
A magnetic recording/reproduction device using such a magnetic head comprises a rotary head type device having a rotating magnetic head, and a fixed head type device having the magnetic head fixed with respect to the longitudinal direction of the tape.
As shown in the fixed magnetic head type magnetic recording/reproduction device of FIG. 1, a magnetic tape 17 travels in a longitudinal direction of the tape (X direction) parallel to a fixed magnetic head 9. At the time of recording, a signal from magnetic head 9 is recorded on a track 19 parallel to the direction of travel of magnetic tape 17. At the time of reproduction, recording track 19 is traced by fixed head 9, whereby the recorded information are reproduced by fixed head (to be more exact, reproduce head) 9.
In the case a plurality of tracks are provided widthwise of magnetic tape 17, recording/reproduction is carried out by: (a) providing a plurality of magnetic heads widthwise of the tape, (b) moving one magnetic head along the direction of the width of the tape, or (c) combining the methods of (a) and (b) to position the magnetic head at a predetermined position widthwise of the tape.
In the cases of (b) and (c), a stepping motor may be used for moving magnetic head 9 widthwise of the tape. A stepping motor provides a rotation by a predetermined angle for every input pulse.
FIG. 2 is a perspective view of a magnetic head moving system employing a stepping motor. Referring to FIG. 2, magnetic head 9 is fixed on one main surface of a head base 10. A nut 12 abuts against the other main surface of head base 10 in a fixed manner. A lead screw 16 of stepping motor 15 is engaged to nut 12. Stepping motor 15 is fixed to a base 14. Lead screw 16 penetrates base 14. Stepping motor 15 is fixed to base 14 so that lead screw 16 is parallel to guideshafts 11a and 11b which will be described afterwards.
Guideshafts 11a and 11b parallel to each other are fixed to base 14. Guideshafts 11a and 11b penetrate respective holes provided in head base 10. Head base 10 is slidable along guideshafts 11a and 11b. Guideshafts 11a and 11b serve as guiding means for moving magnetic head 9 widthwise and parallel to magnetic tape 17 by means of head base 10.
Head base 10 is prevented from moving in a direction perpendicular to the sliding plane of the magnetic tape (the plane opposing magnetic head) by guideshafts 11a and 11b.
With the aid of pressing plate 13, nut 12 prevents the rotary motion of lead screw 16 with respect to head base 10.
Stepping motor 15 rotates lead screw 16 in response to an applied pulse. In accordance with the rotation of lead screw 16, nut 12 engaged to lead screw 16 moves. This movement of nut 12 causes the movement of head base 10 having magnetic head 9 fixed thereto in the width direction of magnetic tape 17. The amount of movement of magnetic head 9 widthwise of the tape per 1 step of stepping motor 15 is expressed by the following equation: ##EQU1## where .theta..sub.s : step angle of stepping motor 15 (deg)
P: pitch of lead screw 16
The step angle of the stepping motor 15 is very precise. By setting lead screw pitch P and step angle .theta..sub.s of the stepping motor to appropriate values corresponding to the amount of the track pitch, magnetic head 9 can move widthwise of the tape to be positioned accurately at a desired track position.
High density recording is required to increase the recording capacity in recent magnetic recording/reproduction devices. A typical method of realizing high density recording is to increase the number of tracks along the width direction of the tape. This will result in a more narrow track width and track pitch.
A flange is generally provided at the widthwise end of a magnetic tape to prevent displacement of the magnetic tape during its travel. Even the provision of such a flange cannot prevent waving during the travel of the magnetic tape, resulting in tracking offset caused by the inability of accurately tracing the appropriate track particularly at the time of reproduction.
When the error between the components in the magnetic head resting position and the error in the mounting position of the record head and the reproduce head widthwise of the track in a magnetic recording/reproduction device are accumulated, the reproduce head deviates from the track to be traced to produce tracking offset.
A guard band is provided between the tracks for preventing crosstalk between adjacent tracks encountered in tracking offset. However, the guard band has a narrow width because of the recent tendency towards narrower track widths and narrower track pitches. There is the possibility of a crosstalk signal in adjacent tracks being mixed into a reproduced signal caused by offset in tracking.
In order to prevent the above-described tracking offset, the still position of the stepping motor must be changed finely according to a position error signal, and not by rotation (or movement) at the step unit. The structure of changing minutely the still position of a stepping motor is disclosed in Japanese Patent Laying-Open No. 62-7398.
This conventional system comprises a position error detecting circuit for obtaining the error between the target position and the current position for each predetermined sampling time interval, an integrator for integrating the detected position error during the sampling period, a digital compensating filter for obtaining a corresponding balancing point of a stepping motor (two-phase linear stepping motor) from the output of the position error detecting circuit and the output of the integrator, and a circuit for applying two-phase current to the stepping motor according to the output of the compensating filter for driving the stepping motor to a corresponding balancing point.
In this conventional system, the linear stepping motor takes advantage of an appropriate combination of an A phase current value and a B phase current value to set the balancing point position to an arbitrary position regardless of the mechanical driving pitch. The compensating filter is used to change the balancing point position according to the position error. This compensating filter requires a shift register, an adder and a multiplier for carrying out Z transformation to obtain the offset of the reference balancing point to a desired balancing point. Therefore, the structure of the device is complex and large.